The cross section of a Lateral Insulated-Gate Bipolar Transistor on SOI (Silicon-On-Insulator) wafer is illustrated in FIG. 1. FIG. 2 shows a simple equivalent circuit of this complex device, which may be described by two equivalent field effect transistors and two junction transistors T1 and T2, wherein the p-well further acts as a source resistor for T2. The LIGBT device looks similar to the Lateral Double-diffused Metal-Oxide-Semiconductor (LDMOS) transistor. The LDMOS transistor is very similar to the familiar NMOS transistor except that the drain contact is separated from the channel region by several microns of lightly-doped material known as the drift region. A schematic LDMOS device is demonstrated in FIG. 3, being similar to the device of FIG. 1 except that the p.sup.+ region referred to as an anode is replaced by a n.sup.+ diffusion, which then is referred to as a drain. In the off-state the LDMOS can use this drift region to support a high drain voltage at the two reverse-biased pn junctions that it forms with the p-well and p-type substrate. This simple modification has a profound effect on the operating characteristics of this device regarding the current-to-voltage characteristics.
In the on-state the LIGBT illustrated in FIG. 1 has an on-resistance which is 5-10 times lower than that of an equivalent LDMOS according to FIG. 3, which makes it a better choice for applications where resistance is of importance. However, the LIGBT has a drawback of having a very high resistance at low currents due to the pn diode at the anode side, which will cause distortion if the transistor is operated at low currents as it will present a non-linear function in this region.
It is possible to design a hybrid IGBT and DMOS device with both n.sup.+ and p.sup.+ diffusions at the anode contact region as illustrated in FIG. 4. This configuration is normally denoted as a Short-Anode LIGBT (SA-LIGBT). With a positive voltage greater than the threshold voltage applied to the gate and a low voltage on the anode this device conducts electron current like an LDMOS device. At a certain current level a voltage drop developed along the p.sup.+ region is sufficient to forward-bias the anode/drift junction and the anode begins injecting minority carriers (holes) into the drift region. These minority carriers modulate the conductivity of the drift region and the device operators like a normal LIGBT. Thus, such a device will have DMOS linear behavior at.sup.31 low currents until the series resistance in the n-drift region will forward-bias the anode p and n junction at higher currents and the device assumes the IGBT current-to-voltage characteristics. Thus the total current-to-voltage characteristics curve may present two fairly linear portions but having quite different derivatives (slopes). Accordingly there will be a transition in the characteristics between the two slopes of the curve where non-linearity will be present and which of course still will produce distortion. (Further information on this may for instance be found in the Technical Report No. ICL 93-020 by Donald R. Disney having the title "Physics and Technology of Lateral Power Devices in Silicon-On-Insulator Substrates", Department of Electrical Engineering, Stanford University, Calif. USA, 1993.)
Besides, if the substrate biasing underneath the insulator in the SOI material is varying, the series resistance in the n-drift region will vary substantially due to the accumulation or depletion of the electrical charge carriers at the insulator and silicon interface, and consequently the current-to-voltage characteristics of the device will vary. Also the transistor will not support the operation under AC conditions since the highest voltage is assumed to be at the p.sup.+ anode.
In some applications such as relay functions in telephone systems there are requirements both of linearity through the origin of coordinates and bi-directional voltage support.
For example U.S. Pat. No. 5,382,535 of 1995, by Malhi and Ng, discloses a "Method of fabricating performance Lateral Double-diffused MOS Transistor", which demonstrates a kind of symmetry and utilizes a technique commonly referred to as RESURF, which stands for REduced SURface Field. However this solution has a centrally extending drain between two source contacts, i.e. the source of one side is not utilized as a drain and vice versa. This means that it can not be considered to be a bilateral solution, but such a component must be connected in series with a corresponding LDMOS to obtain bilateral action.